Tool and its finishing method of surface for center fin member of air conditioning apparatus for vehicle

ABSTRACT

A tool and its finishing method of surface for center fin member of an air conditioning apparatus for a vehicle. The tool includes a plurality of fin blades each having a circular cutter shape and being provided at a periphery thereof with a plurality of cutting teeth and at a central portion thereof with a central hole. The fin blades are stacked on one another in such a fashion that the central holes are aligned together, and a pair of jig discs are arranged in a coaxial and integral fashion with the fin blades. The tool serves to form louvered center fins having a plurality of continued flat wall portions formed by bending a thin strip in a desired pitch in a zig-zag fashion, each of the flat wall portions having a plurality of louvers extending in perpendicular to the flat wall portion. Respective crests are nitrified to a depth of 20 to 60 μm under the condition in which no heat treatment has been conducted for the fin blades, whereby a nitride layer exhibiting a hardness of Hv 1200 to Hv 1300 is diffused in each of the crests.

TECHNICAL FIELD

The present invention relates to a tool for manufacturing louvered finsfor a heat exchanger and a surface finishing method for the tool, andmore particularly to a tool for manufacturing louvered center finsconfigured to achieve an improvement in heat exchange efficiency in avariety of heat exchangers. The present invention also relates to amethod for finishing the surface of such a tool.

BACKGROUND ART

In an air conditioner equipped in vehicles, a heat exchanger serves asan interface for conducting a heat exchange of the air conditioner withambient air. Such a heat exchanger is configured in various forms, forexample, in the form of a condenser, a radiator, a heater core, or anevaporator.

Although there are a variety of heat exchangers, they have similarconfigurations. That is, most heat exchangers include a pair of headertanks for receiving a heat exchange medium introduced therein, and aplurality of parallel heat exchange tubes arranged between the headertanks in a stacked fashion and adapted to form an elongated flow passagefor the heat exchange medium.

A louvered fin having the form of a sheet is interposed between adjacentones of the heat exchange tubes while being bonded to those adjacentheat exchange tubes. For the bonding to the heat exchange tubes, thelouvered fin is coated, at both surfaces thereof, with clad layers madeof a fusible metal having a low melting point.

The louvered fin is bent in a zig-zag fashion to have a corrugatedstructure. In order to provide a maximized heat exchange efficiency, thefin is also provided with a plurality of,parallel louvers so that it hasa louvered structure. The louvers are formed by cutting each flat wallportion of the fin at a plurality of positions along the length of theflat wall portion, and then bending those cut portions from the plane ofthe flat wall portion. By virtue of such a louvered structure, the finhas a maximum contact area with ambient air.

Since the above mentioned fin has a corrugated and louvered sheetstructure bent in a zig-zag fashion and provided with a plurality ofbent louvers, it is necessary to use a specific tool for the manufactureof such a fin.

Conventionally, such a tool includes a plurality of stacked parallel finblades each provided at its periphery with a plurality of cutting teetharranged in a pitch corresponding to the bending pitch of a fin to beformed. The number of the fin blades corresponds to the number oflouvers to be formed at each flat wall portion of the bent fin.

In order to manufacture a louvered fin, a pair of tools having. theabove mentioned arrangement are used. The tools are arranged adjacent toeach other to define a nip therebetween. A metal sheet made of aluminumor clad-coated aluminum exhibiting a high thermal conductivity is forcedto pass through the nip between the tools, so that it is simultaneouslysubjected to a bending process for the formation of corrugations in azig-zag fashion and a cutting and bending process for the formation oflouvers.

The fin blades of the tools are subjected to a severe using condition inthat they are repeatedly and continuously used for the repeated andcontinued bending and cutting processes. For this reason, it isimportant to lengthen the life of the fin blades.

To this end, a variety of proposals have conventionally been made. Forexample, fin blades are manufactured using a high-speed steel thin platewhich is subjected to a quenching process to have a Vickers hardness of700 to 750 and then subjected to a surface treatment using a gaseousnitrification method.

However, conventional surface-treated tools used for the manufacture oflouvered fins for heat exchangers have the following problems:

(1) The tools are easily abraded during the manufacture of louvered finsbecause they rotate at a high speed. For this reason, the tools shouldbe periodically ground;

(2) Since the article to be machined by the tools is made of a metal,such as aluminum, exhibiting a high viscosity in most cases, burrs mayoccur at the tooth crests of the fin blades. As a result, a considerabledegradation in workability occurs;

(3) In particular, where the clad metal of the fin is an alloy materialcontaining Si, for example, a Al—Si-based or Al—Mn—Si-based compositematerial, the tools may be early abraded due to Si exhibiting a veryhigh hardness;

(4) Since each tool uses a plurality of fin blades individuallymanufactured and then stacked together, it is very expensive. For thisreason, a frequent replacement of such a tool results in a considerableincrease in costs; and

(5) The frequent replacement and grinding of the tools cause a frequenttemporary shut-down of the production line. Furthermore, the repeatedgrinding of the tools may result in a variation in the dimensions oflouvered fins initially designed. As a result, it is impossible tomanufacture louvered fins with an optimum heat exchange efficiency.

DISCLOSURE OF THE INVENTION

Therefore, the present invention has been made in view of the abovementioned problems involved in conventional tools used for themanufacture of louvered fins for heat exchangers and conventionalsurface finishing methods for those tools, and an object of theinvention is to provide a tool for manufacturing louvered center fins,which has a maximized life, and a surface finishing method for the toolcapable of allowing the tool to have a maximized life.

In accordance with the present invention, this object is accomplished byproviding A tool for manufacturing louvered center fins of a heatexchanger and a surface finishing method for the tool, the toolincluding a plurality of fin blades each having a circular cutter shapeand being provided at a periphery thereof with a plurality of cuttingteeth and at a central portion thereof with a central hole, the finblades being stacked on one another in such a fashion that the centralholes are aligned together, and a pair of jig discs arranged at oppositesides of the stacked fin blades, respectively, in such a fashion thatthe jig discs are arranged in a coaxial and integral fashion with thefin blades, the tool serving to form louvered fins having a plurality ofcontinued flat wall portions formed by bending a thin strip in a desiredpitch in a zig-zag fashion, each of the flat wall portions having aplurality of louvers extending in perpendicular to the flat wallportion, wherein: respective crests of the cutting teeth in each of thefin blades are plasma-nitrified to a depth of 20 to 60 μm, therebyforming a nitride layer on each of the crests; and depositing a coatingof TiC, TiN, or TICN to a thickness of 2 μm or less over the nitridelayer in accordance with a plasma chemical vapor deposition method or aplasma physical vapor deposition method. Now, the tool for manufacturinglouvered fins of a heat exchanger and a surface finishing method for thetool according to the present invention will be described in detail interms of configurations, functions and effects, as compared with thoseof conventional cases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a condenser which is anexample of a heat exchanger to which the present invention is applied;

FIG. 2 is a perspective view illustrating a heater core which is anotherexample of a heat exchanger to which the present invention is applied;

FIG. 3 is a perspective view illustrating a radiator which is anotherexample of a heat exchanger to which the present invention is applied;

FIG. 4 is a perspective view illustrating a tool for manufacturinglouvered fins of a heat exchanger having a typical configuration, and amanufacturing method using the tool;

FIG. 5 is an enlarged view illustrating the typical louvered finsmanufactured in accordance with the manufacturing method of FIG. 4, andFIG. 5A is a cross-sectional view taken along the line F-F′ of FIG. 5;

FIG. 6 is a perspective view illustrating a configuration of the louverfin manufacturing tool shown in FIG. 4, along with fin blades includedin the tool;

FIG. 7 is an enlarged view illustrating a cutter portion of one finblade included in a conventional louvered fin manufacturing tool;

FIG. 8 is an enlarged view schematically illustrating a cutter portionof one fin blade included in a conventional louvered fin manufacturingtool;

FIG. 9 is a side view illustrating a jig adapted to stack fin blades onone another for a heat treatment in a conventional case;

FIG. 10 is a side view illustrating a jig adapted to stack fin blades onone another for a heat treatment in accordance with the presentinvention;

FIG. 11 is a partial enlarged view of the louvered fin manufacturingtool according to the present invention, in which the figure portion Aillustrates the crest of one fin blade being in an initiallymanufactured state, the figure portion B illustrates the crest being ina degraded state after a repeated use thereof, and the figure portion Cillustrates the crest being in a ground state; and

FIG. 12A is a cross-sectional view of one fin blade being in asurface-treated state in accordance with a conventional surfacefinishing method, and FIG. 12B is a hardness graph of the fin blade ofFIG. 12A, illustrating problems involved in the conventional surfacefinishing method.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a perspective view illustrating a condenser which is anexample of a heat exchanger to which the present invention is applied.FIG. 2 is a perspective view illustrating a heater core which is anotherexample of a heat exchanger to which the present invention is applied.FIG. 3 is a perspective view illustrating a radiator which is anotherexample of a heat exchanger to which the present invention is applied.FIG. 4 is a perspective view illustrating a tool for manufacturinglouvered fins of a heat exchanger having a typical configuration, and amanufacturing method using the tool. FIG. 5 is an enlarged viewillustrating the typical louvered fins manufactured in accordance withthe manufacturing method of FIG. 4, along with a cross-sectional viewtaken along the line F-F′. FIG. 6 is a perspective view illustrating aconfiguration of the louver fin manufacturing tool shown in FIG. 4,along with fin blades included in the tool. FIG. 7 is an enlarged viewillustrating a cutter portion of one fin blade included in aconventional louvered fin manufacturing tool. FIG. 8 is an enlarged viewschematically illustrating a cutter portion of one fin blade included ina conventional louvered fin manufacturing tool. FIG. 9 is a side viewillustrating a jig adapted to stack fin blades on one another for a heattreatment in a conventional case. FIG. 10 is a side view illustrating ajig adapted to stack fin blades on one another for a heat treatment inaccordance with the present invention. FIG. 11 is a partial enlargedview of the louvered fin manufacturing tool according to the presentinvention, in which the figure portion A illustrates the crest of onefin blade being in an initially manufactured state, the figure portion Billustrates the crest being in a degraded state after a repeated usethereof, and the figure portion C illustrates the crest being in aground state. FIG. 12 is a cross-sectional view of one fin blade beingin a surface-treated state in accordance with a conventional surfacefinishing method, along with a hardness graph of the fin blade,illustrating problems involved in the conventional surface finishingmethod.

Referring to FIGS. 1 to 3, respective configurations of louvered finsare illustrated which are used in a variety of heat exchangers and towhich the present invention is applied.

FIG. 1 illustrates louvered fins F each interposed between adjacent onesof heat exchange tubes 2 in a condenser 1 having a typicalconfiguration. FIG. 2 illustrates louvered fins F each interposedbetween adjacent ones of heat exchange tubes 12 in a heater core 10. Inaddition, FIG. 3 illustrates louvered fins F each interposed betweenadjacent ones of heat exchange tubes 22 in a radiator 20. A detailedconfiguration of such louvered fins F is illustrated in FIG. 5.

Each louvered fin F is formed by bending a metal sheet in a zig-zagfashion. Referring to FIGS. 5 and 5A, the louvered fin F is formed bybending a thin strip S in a pitch P in a zig-zag fashion to form aplurality of continued flat wall portions D, cutting each flat wallportion D of the strip S at a plurality of positions uniformly spacedalong the length of the flat wall portion D, and then bending those cutportions from the plane of the flat wall portion D, thereby forming aplurality of parallel louvers L connected together at their ends L1 andL2.

In order to manufacture the louvered fin F having the above mentionedstructure, therefore, it is necessary to use a specific tool capable ofbending a thin strip S in a pitch P in a zig-zag fashion to form aplurality of continued flat wall portions D, cutting each flat wallportion D of the strip S at a plurality of positions uniformly spacedalong the length of the flat wall portion D, and bending those cutportions from the plane of the flat wall portion D, thereby forming aplurality of parallel louvers L. An exemplary configuration of such atool is illustrated in FIGS. 4 and 6.

Referring to FIG. 6, a tool T is illustrated which is adapted tomanufacture louvered fins of a heat exchanger in accordance with thepresent invention.

As shown in FIG. 6, the tool T includes a plurality of fin blades 30each having a circular cutter shape. Each fin blade 30 is provided atits periphery with a plurality of cutting teeth 35 arranged in a pitchTP. The fin blades 30 are fitted around a fixed central shaft (notshown) in a stacked fashion. For the fitting around the fixed centralshaft, each fin blade 30 has a central hole 31. At opposite sides of thestacked fin blades 30, a pair of guide jig discs 40 and 41 are fittedaround the fixed central shaft, respectively. The guide jig discs 40 and41 serve to guide a thin strip S to be formed into a desired louveredfin F. The fin blades 30 and guide jig discs 40 and 41 are fixedlymounted to the fixed central shaft by means of fixing members such asbolts.

In order to manufacture a louvered fin, a pair of tools T having theabove mentioned arrangement are used, as shown in FIG. 4. Referring toFIG. 4, the tools T are arranged adjacent to each other to define a niptherebetween. The tools T are rotated in opposite directions by ahigh-speed rotating mechanism (not shown), respectively. A thin strip Sis introduced into the nip between the facing tools T and fed throughthe nip by the rotations of those tools T. As the thin strip S passesthrough the nip between the tools T, it is bent in a zig-zag fashion toform a corrugated structure while being simultaneously shaped to formlouvers L.

Such simultaneous formation of the corrugated structure and the louveredstructure is achieved by the tools T in which the cutting tooth 35 ofeach fin blade 30 has a pitch TP corresponding to the pitch P of the finF, and the space between adjacent ones of the fin blades 30 correspondsto the space between adjacent ones of the louvers L.

However, the fin blades 30 of the tools T are subjected to a severeusing condition in that they are repeatedly and continuously used forthe repeated and continued bending and cutting processes. For thisreason, the life of the fin blades may be considerably reduced.

In order to solve such a problem, efforts to increase the surfacehardness of the fin blades have conventionally been made. As mentionedabove, a gaseous nitrification method has been used.

However, in accordance with such a gaseous nitrification method, anon-uniform nitrification may occur depending on different positions ofthe article, to be treated, in a nitrification furnace and differentportions of the article. As a result, the surface-treated article mayhave deviations in nitrified depth and surface hardness.

Where a high-speed steel, which is used for the fin blades 30, issubjected to a gaseous nitrification, and Fe₂₋₃C compound layer CM of aundesirable brittle ε-phase may be formed to a certain thickness inaddition to an intended nitrogen diffused layer ND, as shown in FIG.12A. Otherwise, an acicular structure may be formed in the nitride layerND. As a result, the resultant structure may be brittle. This may resultin a formation of chippings during the manufacture of fin blades usingthe surface-treated high-speed steel. Otherwise, the material may bebroken.

As shown in FIG. 12B, it is possible to obtain an increase in hardnessup to Hv 1200 in accordance with the above mentioned gaseousnitrification method. However, there is a problem associated with theabove mentioned brittleness. Furthermore, in this case, it is impossibleor difficult to re-use fin blades manufactured in accordance with theabove mentioned method because chippings may be formed when those finblades are subjected, at side surfaces thereof, to a grinding processfor the re-use thereof. In order to solve such problems involved in theconventional method, the present invention provides a specific tool formanufacturing louvered fins, and a surface finishing method for thetool. Now, the tool and the surface finishing method for the toolaccording to the present invention will be described in detail withreference to the following examples.

EXAMPLE 1

Each fin blade 30 of the tool T is partially subjected to a plasmanitrification under the condition in which no heat treatment isconducted. That is, only the crests E of the cutting teeth 35 in eachfin blade 30 are subjected to the plasma nitrification up to a depth of20 to 60 μm, preferably, 30 to 50 μm, without being subjected to anyheat treatment, in accordance with this example of the presentinvention.

In accordance with this example, the plasma nitrification is carried outin a conventional fashion by filling Ar, N₂, H₂, and CH₄ in a vacuumchamber loaded with workpieces, that is, the fin blades 30, to beprocessed, and maintained at a high temperature, and then conducting adischarge in the vacuum chamber. As the discharge occurs, Ar strikes thesurface of each workpiece, thereby causing Fe existing on the surface ofthe workpiece to be excited. As a result, a diffusion of N into theworkpiece is accelerated. A part of N may react with the excited Fe²⁺,thereby forming an undesirable F₂₋₃N (an ε-phase) and an undesirableFe₄N (γ′-phase). In order to avoid the formation of such undesirable εand γ′-phases, the ratio of N2 and H2 contained in the atmosphere formedin the vacuum chamber is controlled in accordance with the presentinvention. When the ratio of N2 and H2 is ranged from 3:7 to 4:6, anitride layer, which consists only of a nitride diffused layer, isformed in the surface of the workpiece.

In accordance with the above mentioned plasma nitrification method, anitride layer exhibiting a substantially uniform gradient along thedepth of the workpiece is formed, so that it exhibits both abrasionresistance and toughness. The discharge nitrification is conducted at arelatively low temperature of 500° C. or less. Accordingly, it ispossible to prevent a variation in the physical properties of theworkpiece and a deformation of the workpiece.

The reason why the plasma nitrification depth is limited to a range of20 to 60 μm is to achieve an easy grinding for the side surfaces of thefin blade 30. After a repeated use, the crest E of each tooth in the finblade 30 may be blunt, as indicated by the reference character R in theview B of FIG. 11. In order to re-use such a blunt fin blade 30, it isnecessary to grind one side surface of that fin blade, thereby allowingthe crest E of the fin blade to have a sharp edge along the grindingplane GF. Where the plasma nitrification depth is ranged from 20 to 60μm, the grinding process can be easily conducted. In addition to such aneffect for the easy grinding process, the plasma nitrification depthlimited to the above mentioned range provides a great reduction in thenitrification cost.

As the crests E of each tooth in the fin blade 30 is subjected to theplasma nitrification, they exhibits a hardness increased up to Hv 1200to Hv 1300. The nitride layer formed in the surface of the fin blade 30consists only of a nitrogen diffused layer and a micro-precipitationphase of Fe₄N. In accordance with the plasma nitrification, theformation of surface compound layers of an ε-phase or acicular compoundsof ε and γ′-phases are substantially inhibited. Accordingly, it ispossible to obtain fin blades having tooth crests E exhibiting a highhardness while being free of physical properties associated withbrittleness.

For the plasma nitrification only for the tooth crest E, a jig 50 havinga central shaft 51 is used, as shown in FIG. 10. A plurality of finblades 30 to be subjected to the plasma nitrification process are fittedaround the shaft 51 of the jig 50 in such a fashion that they arestacked on one another without any space between adjacent ones thereof.Where the fin blades 30 mounted to the jig 50 in the above mentionedfashion are simply loaded in the vacuum furnace, only their peripheralsurfaces, namely, tooth crests, are exposed to the atmosphere in thevacuum furnace. Accordingly, the plasma nitrification can be easilyconducted only for the exposed tooth crests E.

Referring to FIG. 9, a jig is illustrated which is used in conventionalgaseous nitrification methods. The jig, which is denoted by the samereference numeral as that of the jig used in accordance with the presentinvention, that is, 50, includes a central shaft 51, and spacers 52. Inorder to ensure the entire portion of each fin blade 30 to besufficiently nitrified, the spacers 52 are used which serve to spaceadjacent ones of fin blades 30, stacked on one another along the shaft51, apart from each other. As compared to the conventional gaseousnitrification method using such a jig, therefore, the plasmanitrification method according to the present invention provides effectsof an improvement in productivity and workability.

EXAMPLE 2

Each fin blade 30 of the tool T is partially subjected to a plasmanitrification after a desired heat treatment is conducted. That is, onlythe crests E of the cutting teeth 35 in each fin blade 30 are subjectedto a heat treatment, and then subjected to the plasma nitrification upto a depth of 20 to 60 μm, preferably, 30 to 50 μm in accordance withthis example of the present invention. In accordance with this example,it is possible to greatly enhance the mechanical strength of the finblade 30.

This example can be used even in the case in which the material of thecutting teeth 35 exhibits a low hardness.

EXAMPLE 3

Each fin blade 30 of the tool T is partially subjected to a plasmanitrification under the condition in which no heat treatment isconducted. That is, only the crests E of the cutting teeth 35 in eachfin blade 30 are subjected to the plasma nitrification up to a depth of20 to 60 μm, preferably, 30 to 50 μm, without being subjected to anyheat treatment, in accordance with this example. The plasmanitrification is carried out in the same fashion as in the first exampleof the present invention.

In accordance with this example, a plasma chemical vapor deposition(CVD) process or a plasma physical vapor deposition (PVD) is conductedfor a plasma nitride layer formed in the surfaces of the tooth crests Eof each fin blade 30, in order to achieve a coating treatment for thosetooth crests E. That is, TiC, TiN, or TiCN, which exhibits a very highhardness, is deposited to a thickness of 2 μm or less over the toothcrests E in accordance with the plasma CVD or PVD method.

As a result of experiments, it could be found that the TiC, TiN, or TiCNcoating deposited over the plasma nitride layer serves to increase thesurface hardness of the tooth crests to Hv 2000 or more.

The reason why the thickness of the TiC, TiN, or TICN coating is limitedto 2 μm or less is because that coating may be deposited in the form ofa spherical structure resulting in a undesirable degradation in cutability.

Where the TiC, TiN, or TiCN coating is directly deposited over the toothcrests under the condition, in which the above mentioned plasmanitrification is not conducted, it may be easily peeled off because anionospheric layer may be formed at the interface between the surfaces ofthe tooth crest and the coating due to a great hardness differenceexhibited between the surfaces of the tooth crests and the coating. Forthis reason, it is necessary to form a relatively thick plasma nitridelayer having a hardness ranged between those of the tooth crests andcoating.

Referring to FIG. 8, it can be found that a plasma nitride layer PLexhibiting a hardness of Hv 1200 or more and a coating CL exhibiting ahardness of Hv 2300 or more are formed over the cutting teeth 35 whichexhibits a hardness of Hv 1200 under the condition not subjected to anysurface treatment.

Industrial Applicability

As apparent from the above description, the tool for manufacturinglouvered fins and the surface finishing method for the tool according tothe present invention can achieve an increase in the life of the toolwhile allowing a grinding of the tool after a repeated use thereof,thereby allowing a re-use of the tool. The present invention alsoprovides an improvement in treatment efficiency, thereby achieving animprovement in utility.

What is claimed is:
 1. A method for finishing a surface of fin blades ofa tool for an air conditioning apparatus, the tool including a pluralityof fin blades each having a circular cutter shape and being provided ata periphery thereof with a plurality of cutting teeth and at a centralportion thereof with a central hole, the fin blades being stacked on oneanother in such a fashion that the central holes are aligned together,and a pair of jig discs arranged at opposite sides of the stacked finblades, respectively, in such a fashion that the jig discs are arrangedin a coaxial and integral fashion with the fin blades, the tool servingto form louvered center fins having a plurality of continued flat wallportions formed by bending a thin strip in a desired pitch in a zig-zagfashion, each of the flat wall portions having a plurality of louvers,the method comprising the step of: nitrifying only a crest portion ofthe cutting teeth in each of the fin blades to a depth of 20 to 60 μmsuch that a nitride layer exhibiting a hardness of Hv 1200 to Hv 1300 isdiffused in each of the crests.
 2. The method according to claim 1,wherein the step of nitrifying the crests is carried out by fitting thefin blades around a central shaft included in a jig in such a fashionthat the fin blades are stacked on one another without any space betweenadjacent ones thereof, and then loading the fin blades mounted to thejig in a nitrifying furnace.
 3. The method according to claim 1, whereinthe step of nitrifying is carried out to a depth of 30 μm to 50 μm. 4.The method of claim 1, wherein said step of nitrifying is performedunder conditions in which no heat treatment has been previouslyconducted for said fin blades.
 5. The method of claim 4, wherein saidstep of nitrifying is with plasma nitrification.
 6. The method of claim5, further comprising, after the step of nitrifying with plasmanitrification, the step of depositing a coating of TiC, TiN or TiCN to athickness of not more than 2 μm over the nitride layer.
 7. The method ofclaim 6, wherein said step of depositing is performed using a plasmachemical vapor deposition method or a plasma physical vapor depositionmethod. 8.The method of claim 5, wherein said step of plasma nitrifyingonly the crest portions is performed by fitting the fin blades around acentral shaft in a jig such that said fin blades are stacked on oneanother without any space between adjacent ones thereof.
 9. The methodaccording to claim 5, wherein the step of nitrifying is performed in avacuum chamber in which a ratio of N₂ to H₂ is controlled to range from3:7 to 4:6.
 10. A method for finishing a surface of fin blades of a toolfor an air conditioning apparatus, the tool including a plurality of finblades each having a circular cutter shape and being provided at aperiphery thereof with a plurality of cutting teeth and at a centralportion thereof with a central hole, the fin blades being stacked on oneanother in such a fashion that the central holes are aligned together,and a pair of jig discs arranged at opposite sides of the stacked finblades, respectively, in such a fashion that the jig discs are arrangedin a coaxial and integral fashion with the fin blades, the tool servingto form louvered center fins having a plurality of continued flat wallportions formed by bending a thin strip in a desired pitch in a zig-zagfashion, each of the flat wall portions having a plurality of parallellouvers, the method comprising the steps of: subjecting only a crestportion of the cutting teeth in each of the fin blades to a hardeningheat treatment; and plasma-nitrifying said heat treated crest portionsto a depth of 20 to 60 μm, such that a nitride layer exhibiting ahardness of Hv 1200 to Hv 1300 is diffused in each of the crests. 11.The method according to claim 10, wherein the step of plasma nitrifyingthe crests is carried out by fitting the fin blades around a centralshaft included in a jig in such a fashion that the fin blades arestacked on one another without any space between adjacent ones thereof,and then loading the fin blades mounted to the jig in a nitrifyingfurnace.
 12. The method according to claim 10, wherein the step ofplasma nitrifying is carried out to a depth of 30 μm to 50 μm.
 13. Amethod for finishing a surface of fin blades of a tool for an airconditioning apparatus, the tool including a plurality of fin bladeseach having a circular cutter shape and being provided at a peripherythereof with a plurality of cutting teeth and at a central portionthereof with a central hole, the fin blades being stacked on one anotherin such a fashion that the central holes are aligned together, and apair of jig discs arranged at opposite sides of the stacked fin blades,respectively, in such a fashion that the jig discs are arranged in acoaxial and integral fashion with the fin blades, the tool serving toform louvered center fins having a plurality of continued flat wallportions formed by bending a thin strip in a desired pitch in a zig-zagfashion, each of the flat wall portions having a plurality of louvers,the method comprising the steps of: plasma nitrifying only a crestportion of the cutting teeth in each of the fin blades to a depth of 20to 60 μm thereby forming a nitride layer on each of the crests; anddepositing a coating of TiC, TiN or TiCN to a thickness of not more than2 μm over the nitride layer in accordance with a plasma chemical vapordeposition method or a plasma physical vapor deposition method. 14.Themethod according to claim 13, wherein the step of plasma nitrifying thecrests is carried out by fitting the fin blades around a central shaftincluded in a jig in such a fashion that the fin blades are stacked onone another without any space between adjacent ones thereof, and thenloading the fin blades mounted to the jig in a nitrifying furnace. 15.The method according to claim 13, wherein the step of plasma nitrifyingis carried out to a depth of 30 μm to 50 μm.